Cache does not get wiped when --skip-magic-trailing-comma changes

[JelleZijlstra]

Describe the bug The cache is still used after you change a configuration option.

To Reproduce Steps to reproduce the behavior:

1. Format some code with Black
2. Change some formatting flag (e.g., add skip-magic-trailing-comma = true to your pyproject.toml)
3. Run Black on your code again and see that nothing changes
4. Figure out where Black stores the cache and rm -rf it
5. Run Black on your code again and see that it changes formatting

Does this bug also happen on master? Yes

This is because FileMode.get_cache_key doesn't include this flag.